This invention relates generally to improved well-logging techniques and more specifically to novel methods of determining channels in the cement behind a well casing utilizing the temperature profiles of the well.
It has become common practice in oil and gas drilling operations to set casing within a well to stabilize the walls of the borehole and to cement the several subsurface formations. When this casing is initially placed within the borehole space exists between the outer surface of the casing and the walls of the borehole. Cement is pumped into this space to provide solid fill and to hold the casing in place. Ideally no large void spaces are left between the casing and borehole wall after the cementing process.
It has also become common practice in the completion of oil and gas wells to perforate the casing, cement and surrounding formations to bring a well into production. If, however, a channel exists in the cement behind the casing, unwanted production may result. For example, water may flow from a water sand through a channel within the cement to a perforated interval, pass through the perforations and enter the production flow. Once the existence of this channel is confirmed, the channel can be blocked by a cement squeeze thereby eliminating the unwanted production.
Several techniques are currently available for identifying channels within cement including radioactive tracer logging, acoustic logging and qualitative temperature profile logging, each of which has its shortcomings. Radioactive tracer logging generally requires multiple logging passes, and the results often depend on the distribution of the tracer and the background radiation. Acoustic logging around the perforations may have sensitivity problems due to the noise caused by production, especially gas production. Qualitative temperature profile logging requires that qualitative judgements be made from the appearance and comparison of several temperature profiles in the various qualitative temperature profile logging methods.
These and other disadvantages are overcome by the present invention which provides a method of determining a quantitative rate coefficient based on temperature profiles, the plot of this rate coefficient versus depth being very responsive to the existence of a channel in the cement behind a well casing. This rate coefficient may further be calculated for a well-bonded interval of the well casing and used to predict temperatures at a zone of investigation. Anomalies in the predicted temperatures as compared to the measured temperatures are further indicative of channeling in the cement behind the well casing.